tmpfiles: also prefix C source paths with --root

tmpfiles: also prefix C source paths with --root

This makes the code match the docs for --root ("all paths will be prefixed").
I think this is reasonable, because --root also works for config paths, and
any configuration inside --root must refer to paths under --root. If we allowed
C to go "outside of root" in this way, the effect of calling systemd-tmpfiles --root=...
and chrooting first and then calling systemd-tmpfiles second would be quite different.
I think it's better to keep things simple and consistent.

Fixes #12467.